This disclosure relates to methods of reactive ion etching and in particular, but not exclusively, to improvements in the Deep Reactive on Etching (DRIE) process and similar processes.
In the field of Micro Electrical Mechanical Systems (MEMS), anisotropic etching of silicon and other substrates can be achieved using a DRIE method referred to commonly as the “Bosch Process”. This process is described for example in U.S. Pat. No. 5,501,893 and involves alternating between a silicon plasma etching step (typically using SF6) and a passivation step including a fluoropolymer (typically C4F8). During the passivation step, the fluoropolymer is deposited on all of the sample surfaces, During the etching step, ion assisted plasma etching is used preferentially to remove the fluoropolymer from the bottom of an etched feature, whilst still retaining its protection on the side walls. The exposed silicon at the bottom of the feature can then be etched, and the process be repeated until the desired depth is reached.
Inherent to DRIE is a phenomenon referred to as Aspect Ratio Dependant Etching (ARDE), in which the etch rate is inversely proportional to the aspect ratio, defined herein as the ratio of depth to width of the formed feature. This leads to the observation known as RIE lag, whereby smaller features etched at the same time are shallower than larger features. When etching a variety of feature geometries down to an etch-stop layer, it becomes necessary to over etch the wider features, This can lead to effects such as “footing” (or notching), as well as a loss of dimensional control.
This is of particular concern in the production of MEMS sensors such as accelerometers. An example of a MEMS accelerometer is disclosed in WO 2012/076837 A1. In this device, a silicon wafer is micromachined to provide a movable proof mass having a plurality of fingers which interdigitate with fingers of a part of the wafer which will, in use, be fixed. The gap between one side of any given proof mass finger and the adjacent fixed finger is different from the gap between the other side of the proof mass finger and its adjacent fixed finger. As the proof mass moves, the gap between the proof mass fingers and the fixed fingers varies, which leads to a change in capacitance between the fingers. This can be measured and processed to calculate an acceleration. In closed loop systems, the variation in gap size produces an electrostatic force which counters the movement of the proof mass. In such systems, it is particularly desirable to have a large ratio of the width of the wider gap (for example 15 microns) to the width of the narrower gap (for example 5 microns). Relatively large ratios can be produced using existing DRIE processes, but at the expense of enlarged finger pitch, making the device larger to provide a given electrostatic force. If the device is made smaller, the above identified problems of footing, notching and loss of dimensional control may occur.
Current proposals to compensate for lag involve optimising the etch and passivation step and parameters, mainly by lowering pressure values. However, current techniques for compensation of DRIE lag are at the expense of reduced etch rates.